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I have studied lead acid battery charging methods from the website http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery. In which they mention three stages of charging:

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My question: I already made a PWM solar charger unit using the P-MOSTFET IRF4905, in which I can control the PWM duty cycle and can be reduce the current, but when reducing the current the voltage applied to the battery will also be reduced, then how can I implement these three stage of charging by use of PWM? i.e. constant current mode ok, but how can we apply top charging?

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You just need to measure both the charging voltage and the charging current. During the first charging phase you adjust the PWM duty cycle so the charger is current limited at its maximum current. That is, you measure both the current and voltage but the duty cycle is adjusted to maintain the desired current. Meanwhile, you monitor the voltage and once the voltage rises to the desired voltage (say 2.4V per cell for lead-acid) you switch to constant-voltage charging. Now you adjust the duty cycle to maintain a constant voltage. Meanwhile, you monitor the current and when the current falls below the level that indicates a full charge you move into the final charging stage. The third stage is also a constant-voltage mode, but at a much lower voltage.

I designed and built a charger that does this. Schematics and source code can be found at my Wordpress pages. It's based on the NXP LPC1114, a Cortex-M0 microcontroller. I didn't use solar panels for the source of power but I don't see why you couldn't reuse most of what I did.

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